I always wear a helmet. Always always. It can't hurt and it might end up saving your life. I have to admit I don't really understand why anyone wouldn't...you can be the safest, most aware, slow moving cyclist in the world but that doesn't mean accidents can't happen. You can't always control your environment and you definitely can't control the people around you. And everyone has moments when their attention wavers a bit, cyclists and drivers both. It only takes a second.
If you land wrong and hit your head on a curb, for instance, you could be in big trouble. I went to high school with a kid who was sitting in the back of a pickup truck, got bounced out, fell 5 feet, hit his head wrong, and unfortunately passed away almost immediately. I try not think about it a lot because obviously it's not a pleasant memory, but it always shoots to mind when I'm on the subject of how important a helmet can potentially be.